How do you go about mic'ing a guitar amp for live performance? Would you do anything different if you were mic'ing the amp for studio recording? Is there anything specific I should look for in a microphone for either of these scenarios?

Why do you want to mic an amp? Isn't the point of an amp to provide volume? Either way, yes, you'd definitely want different set-ups for studio vs live as you have very different environments, acoustically speaking.
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AnonymousJan 14 '11 at 3:38

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@kaunartist Some people want as authentic a tone or as close to live as possible, not always possible by other means
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DRLJan 14 '11 at 5:38

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"Why do you want to mic an amp?" Because an amp is part of the signal chain of an electric guitar, and, especially in rock and blues, can have a big effect on the overall guitar sound. Projecting that sound off the stage accurately means miking the speakers and using the house PA to accurately reproduce it for the audience.
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AnonymousJan 14 '11 at 5:58

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@kaunartist: "To provide volume" is only part of the point of an amp. Indeed, for many many guitarists, the point of an amp is more about tone than volume. For example, I have a vintage '68 Fender Princeton Reverb, which is too small not to use a mic in a live setting. It's tone is amazing, though, so I prefer it to my much louder Twin Reissue. It doesn't hurt, too, that the Princeton is much smaller and lighter to carry.
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Alex BassonJan 14 '11 at 11:50

Place guitar rig in the middle of the room. Position so the back of
the amp is 3 feet from the back wall with the speakers facing the
room.

‘Close’ Micing using the SM-57s

SM57 are flat and beefy. Using these as ‘close mics’ will give a
punchy mid/low sound.

Place the first SM-57 pointing at the left wall of the speaker. Put
this mic 3 inches away from the speaker’s grill.

Place the second SM-57 pointing straight at the speaker’s cone. Put
this mic 10inches away from the speaker’s grill. (you may have to
slightly turn up the gain on this mic.

IMPORTANT: place a sound baffle between the 2 mics. A small pillow
will do perfectly. This will cancel out much “common audio” between
these 2 mics and thus increase the stereo image and decrease any
phasing.

‘Far’ Micing using the Condenser Mics.

Condenser mics are bright, shiny and well defined. Using these as ‘far
mics’ will give a clear, wide open spatial sound with a lot of
definition.

Place each condenser mic in the corners of the room facing the amp.
These 2 mics should form a triangle with the amp - with the amp being
at the apex.

If you can, put a divider/sound bat (6 feet high, 4 feet wide, 2inches
thick) 5 feet in front of the speaker. Position it so it’s thin edge
points at the speaker. Having a wall in the middle of the room will
increase the stereo separation of the 2 condensers.

An image to help explain this:

Check out his blog - he does give a lot of info on how they record various things, and how they do the synths they use extensively.

You could probably write a whole book on this subject; certainly people have written whole chapters.

At the end of the day, it's a matter of "what sounds good, works", and electric guitar players have so many different sounds, there's no one-size-fits-all solution.

Based on reading, not personal experience:

Mic choice

Dynamic mics seem to be the basic workhorse of this job. People like SM-57s.

Ribbon mics might sound lovely, if you can afford them, in the studio. They're probably too fragile, and the benefits too subtle, to be suitable for a lot of live work (although, if you have a nice sedate gig in an orchestra pit or something this might work OK).

Mic placement

With one mic, you already have a range of choices. Perpendicular to the cone, you can point it right at the middle, or on the edge of the cone, or any point in between. As you move outward, the sound will get less bassy and more trebly.

Then, you can change the angle of the mic relative to the cone, to get sound from a wider spread of the cone. The only real way is to listen as you go.

You can use two mics in a combination of the positions described above, and blend them together. Try with the mics both in and out of phase (button on the mixer) -- it will make a huge difference, but only you know which you prefer.

Some people like a second mic on the inside of the cabinet, pointing at the back of the cone.

In the studio, you can place a second mic further back, to capture the room ambience. This isn't likely to work live.

The SM58 makes a good microphone for recording, both live and in the studio.
(If you don't like the ball cap on the SM58, just unscrew it, and then it is exactly the same as the previous SM57).
Set up the mic on a stand, and position the cap of the mic about 2-3 inches from the mesh that covers the speaker. Many people have their own preferences when it comes to positioning around the speaker, and whenever I mic a stand, I usually place the mic at the 8 or 4 O'clock position.

Where the mic goes in relation to the centre of the speaker is quite important.

To close to the centre of the cone and the mic will record a sound that is too bassy.

Too near the edge of the cone and the mic will record a sound that is too trebley.

What you want to find is the 'sweet spot' you hear engineers talk about. This will of course vary with whatever sound you are trying to record.

When amplifying for live performance, I would suggest placing the mic near the middle of the radius of the cone, and then you can adjust the sound from the Amp's EQ controls.

Besides miking to accurately reproduce the sound of your amp/speakers, you're also trying to isolate your guitar sound from the rest of the band, so mike placement has to take that into account.

Don't point a mike toward another instrument's amp or toward the drummer, because bleed-through from the other sound source can mess up your sound capture, and make it harder on whoever is running your mix. So, if you put a mike off-axis from the speaker, keep it reasonable.

If you have a good on-stage monitor system, you might want to consider putting your cabinet off-stage with a mike (or mikes) so you can crank the amp head without blowing your ears out. Having it offstage isolates the sound well, then mixing your miked sound into your monitor allows you to hear. The only downside to it is you don't want the speaker and mike in a room where people can get at them. Imagine your surprise to pick up a conversation over that feed.

There are lot of ways; if you have and amp/cabinet with more than one speaker cone; mic up the best sounding one; one or two dynamic mics will sound very different in different areas of the cone so mess around with it; also; when dialling in your tone on the amp; get your ear as close the the speaker as possible; so you can hear what the mic(s) hear (its very different from the the sound you would hear standing away from the amp so you'll have to adjust accordingly to get the tone you what recorded). Some people also stick a mic around the back of the amp.

Personally, I would never mic an amp. It doesn't make any sense to me when you could just run a line from the amp (or straight from the guitar/box) into your recording equipment and have higher audio quality.

You might want to ask this question on Audio.SE as well, they may provide better answers.

It makes a lot of sense when you remember that the cabinet and speaker act as filters for the amp's output, modifying the sound of the guitar in ways that even modeling amps have a hard time imitating. I get a good sound from my Pod X3, but it isn't the sound that a good tube amp will get through some nice speakers. Even in the studio they'll mike the cabinet and speakers several ways, in addition to a direct-out to provide a mix of all the sounds.
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AnonymousJan 14 '11 at 6:02

Definitely that's a problem. Direct out, or tapping the speaker output, is fine for clean tones, but sounds like a fuzz box with amp distortion unless it's heavily EQ'd to cut the highs, or filtered with some sort of speaker emulator. 12" speakers are not high-fidelity devices and they seriously change the sound of the guitar and amp, but it's a sound we're accustomed to and want.
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AnonymousJan 14 '11 at 17:54

@the Tin Man: I can see your point. This is just my preference :)
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Matthew Read♦Jan 14 '11 at 19:15

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I understand your preference and why too. I've sat behind the console, both live and in the studio, plus played on stage and in the studio, so I know the need for clean feeds plus the sonically-altered sound from an amp/cabinet combo. Some day, modeling technology will be indistinguishable from tubes to the guitarist, and most of the reason for an amp on stage will go away. My Pod X3 has a couple sounds I'd happily use live, straight into the board, both for clean and distorted songs. Unfortunately it doesn't respond quite like a tube amp, and that irritates me.
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AnonymousJan 14 '11 at 19:49

That said, there are a couple of tried-and-true techniques. For live performance, you can't do much better than a simple Shure SM-57 in front of your best-sounding speaker, maybe off-axis if you prefer that sound. 57's sound great and handle the bumps and bruises of live gigs exceptionally well.

... but be careful with the mic capsule; most of the mic is very durable billeted aluminum/steel, but the free-floating capsule enclosure can't take as much as, say, a 58's grille. It's why I prefer the Audix i5 for snare, just because the grille-enclosed design can handle a few whacks from an overzealous drummer. Guitar cabs, you don't have that problem
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KeithSJan 17 '13 at 19:25